1. Field of the Invention
The present invention relates to a tightening structure for performing tightening using high-strength self-forming screws (bolts).
2. Description of the Related Art
In a conventional tightening structure for tightening a peripheral site of an engine using high-strength bolts, the high-strength bolt is screwed to an internal (female) screw of a nut or an internal screw tapped into a partner member. An internal thread portion of a nut or a tapped internal screw hole is of course formed by a cutting process. When an internal screw hole is formed by performing a tapping process on an aluminum die-cast member, a chassis or suspension member, a forged member, or the like in a peripheral site of an engine, for example, precision is required in the hole diameter, and therefore a large number of manufacturing steps, including casting (or prepared hole forming by forging), prepared hole drilling, prepared hole washing, tapping, and screw hole washing must be performed on the die-cast member or other partner member, as shown in FIG. 7.
Further, attempts have been made to increase the strength and reduce the size of the bolt in order to reduce vehicle weight and achieve tightening with a greater tightening force. This point is a well-known technical problem, as described in Japanese Unexamined Patent Application Publication 2005-29870 and so on, for example. Accordingly, advancing the use of high-strength bolts having a strength of 14T (minimum tensile strength 1400 N/mm2, hardness 44 to 47 HRC), which is the maximum strength that can be put to practical use at present, in place of conventional bolts to achieve dramatic size reductions in comparison with conventional bolts has become an unavoidable problem in terms of fuel efficiency, environmental friendliness, and so on. Accordingly, in recent years, the development of bolts having maximum strength has become a technological problem requiring urgent attention.
However, to achieve a strength increase and a size reduction in a bolt, the size of the nut member must be reduced in accordance therewith, and therefore the size of a thread ridge in the internal screw hole of the nut member must also be reduced in size. If the bolt alone is increased in strength, a load on a thread ridge fitting portion of the nut member increases, leading to a large offset load on a root portion of a first fitting thread of the nut member, which may lead to a fatigue fracture in a corresponding location. In addition to fatigue fractures, the nut member may be unable to withstand a tightening force of 14T, which is applied during static tightening performed to achieve tightening, and as a result, the internal screw part may break. Therefore, the fitting portion of the thread ridge to which the nut member is screwed must be lengthened (in the case of an aluminum internal screw, between 2.5 and 3 times the screw diameter, and in the case of a steel internal screw, no less than 2.2 times the screw diameter) to reduce the load value per thread. In such a case, large-scale design modification must be performed on the periphery of the screw hole of the partner member doubling as the internal screw member in order to increase the hole depth, and since the fitting length (fitting portion) becomes longer than a conventional fitting length as a result, a retrograde step is made in terms of reducing the size of the bolt.
Furthermore, attempts have been made in the related art to improve fatigue strength by rolling the bolt following heat treatment to provide the root portion with residual stress, thereby improving the fatigue strength. For example, it is said that an engine bolt can withstand use when the fatigue strength thereof at the number of stress cycles of 5×106 is approximately 50 MPa, and therefore the actual fatigue strength of the engine bolt must possess at least this characteristic. A fatigue strength of 50 MPa is the average ability level of a product obtained through heat treatment after rolling. However, these elements vary according to relative pitch errors with the combined nut, the spring constant of a bolt/nut fastener, and tightening axial tension variation, and therefore rolling after heat treatment is performed to obtain a greater increase in the fatigue limit. However, when rolling after heat treatment is performed on a high-strength bolt having a strength of 14T (minimum tensile strength 1400 N/mm2, hardness 44 to 47 HRC), a rolling tool is subjected to severe wear, and as a result, the life of the tool decreases to 1/10 of a tool used in heat treatment after rolling. It is therefore impractical in terms of productivity and cost to improve the fatigue strength of a bolt having a strength of 14T by performing rolling after heat treatment. Japanese Unexamined Patent Application Publication 2007-321858 discloses another method of improving fatigue strength in which a pitch error between a bolt and a nut is used to reduce the degree of unevenness in the distribution of stress on a fitting screw root portion. However, the value of the pitch error used in this method must be managed in μm units, and therefore this method is likewise impractical in terms of productivity and cost. Hence, at present, no effective solutions exist for increasing the fatigue limit.